Electrodes for use in electrochemical cells such as fuel cells are well known in the art. One common type of fuel cell electrode is the gas diffusion electrodes. In a fuel cell, a pair of gas diffusion electrodes are disposed on either side of a compartment containing a matrix soaked with an electrolyte. A catalyst is disposed on the electrolyte facing surface of each electrode, with hydrogen or another gaseous fuel fed to the back side of one electrode while oxygen or air is fed to the back side of the other electrode. The gases enter the electrodes and react with the electrolyte in the presence of the catalyst.
Many types of gas diffusion electrodes are described in the literature. One type of gas diffusion electrode comprises a layer of polytetrafluorethylene (PTFE) mixed with a catalyst supported on carbon particles (i.e., catalyzed carbon), the layer being disposed on the surface of a porous carbon substrate (e.g. carbon paper). The PTFE (or any other compatible hydrophobic polymer) prevents the electrolyte from filling up the electrode to such an extent that sufficient fuel or oxidant cannot reach the catalyst. A gas diffusion electrode of this type is described in commonly owned U.S. Pat. No. 3,857,737 to Kemp et al.
Another type of gas diffusion electrode comprises a carbon paper substrate with a layer of uncatalyzed carbon mixed with PTFE applied to the surface thereof. Unsupported catalyst is then applied to the carbon/PTFE layer. An electrode of this type is described in commonly owned U.S. Pat. No. 3,972,735 to Breault.
Several methods are known in the industry for adding the catalyst layer to the electrode. One technique for applying the catalyst layer to the substrate is the one step direct filtration method, where a catalyzed powder is mixed in an aqueous dispersion and blended to form a co-suspension of carbon and PTFE. The suspension is caused to floc, such as by heating or adding a floccing agent, with floccing involving the coalescence of catalyzed or uncatalyzed carbon particles with the PTFE particles. A proper floc is one which achieves uniform size agglomerates of catalyzed or uncatalyzed carbon and PTFE particles and a homogenous distribution or mix of the particles. After floccing, excess liquid is decanted and the floc is applied to the surface of a porous substrate which acts as a filter. Liquids pass through the substrate, leaving a layer of the desired floc on the surface. The article is then compacted, dried and sintered to form an electrode.
One problem with the one step addition and filtration of the aqueous floc directly onto the substrate is the production of a rough, cracked catalyst surface. As liquid is filtered from the deposited solids mixture, mud cracks develop as the catalyst layer shrinks. Compaction smooths the surface, but does not relieve the internal shrinkage stresses, particularly in relatively thick catalyst layers. Consequently, during drying, further shrinkage occurs and macrocracks form which extend into the layer, providing potential sites for matrix failure and short circuiting within a fuel cell.
Another technique for applying a catalyst to an electrode substrate utilizes a dry floc method. In U.S. Pat. No. 4,233,181 to Goller et al., a method is disclosed which describes mixing of a catalyzed or uncatalyzed carbon powder in an aqueous PTFE dispersion which is then blended into an alcohol water solution to form a co-suspension of carbon and PTFE. The suspension is caused to floc, such as by heating or adding a floccing agent, and the excess liquid is removed by filtration. The wet flocc is then dried and pulverized to a fine powder and applied to the surface of an electrode substrate by dispersing the powder as a cloud in a chamber over the substrate and pulling the powder into the substrate by drawing a vacuum under the substrate. The powder is then compacted and sintered.
Several problems have been experienced in the handling and storage of the powdered dry floc material, particularly in the efforts to use the dry floc powder in an automated electrode manufacturing procedure. The dry floc powder tends to agglomerate into larger particle sized due to the PTFE components of the floc and is difficult to work with since the powder does not flow freely from hoppers and tends to stick to the surfaces of automated materials handling equipment. Consequently, a method is needed for producing electrochemical cell electrodes in a faster more economical production process while minimizing mud cracking and material handling problems.